1. Field of the Invention
The present invention relates to characterization of a flat surface. More specifically, the present invention pertains to the use of an optical interferometer to analyze the flatness of a flat surface. Further still, the present invention presents an apparatus and method for profiling a smooth surface, such as the surface of a magnetic recording disc.
2. Description of the Related Art
The computer industry employs magnetic discs for the purpose of storing information. In this respect, computer systems employ disc drive systems for transferring and storing large amounts of data between magnetic discs and the host computer. The magnetic discs are typically circular in shape, though other shapes are used. One or more discs may be used in a disc drive system, depending on the needs of the system and the capacity of the drive.
It is desirable that the surface of a magnetic disc be as flat as possible. Uniform flatness aids in maintaining a constant fly-height of the slider, where the magnetic read/write head operates over the disc surface. This, in turn, ensures accurate writing/reading of magnetic data by the read/write head to and from the disc. Flat surface topography also allows the slider and attached magnetic head to fly more closely to the disc surface, permitting a tighter concentration of magnetic data to be embedded in the disc. Thus, periodic surface characterization of magnetic discs is part of the quality control employed in the manufacturing process.
In order to accurately analyze surface topography in ultra-smooth surfaces, it is known to use an optical interferometer. An optical interferometer is a tool that provides the unique advantages of non-contact operation, high resolution, wide spatial frequency coverage and high throughput. However, conventional interferometers are extremely sensitive to environmental vibration.
In a conventional interferometer, the surface topography is inferred by measuring the optical path length difference between an object beam and a reference beam. The reference beam length is usually fixed to be a constant length. Environmental vibration can cause body movement between the interferometer and the test object, which in turn can introduce a spurious change of optical path length in the object beam. Stated another way, if the disc surface experiences vertical vibration, the optical path length difference between the object and reference beams can no longer be kept constant. This vibration-induced optical path length change will then be confounded with the signal of interest originating from the surface topography of the test object, e.g., a magnetic disc surface.
An effective solution to reduce the effect of environmental vibrations in interferometers is to translate the optical path length change caused by body movement into both the reference beam and the object beam. Such interferometers are known as common-path interferometers. There are three types of common-path interferometers; the heterodyne interferometer, the interferometer with a bifringent lens, and the scanning shearing interferometer. A common feature of these interferometer designs is the use of a single lens to deliver two beams to the object surface. The two beams are offset in striking the target surface. The two beams are typically generated by using a birefringent lens or a Wollaston prism. However, in these designs the separation distance “d” between the two beams as they strike the target surface is fixed. Moreover, the separation distance is limited by the numerical aperture of the lens and/or Wollaston prism. The maximum measurable spatial frequency is subsequently limited by these components.
Therefore, a need exists for an optical interferometer that insures a constant optical path length difference between the object and reference beams while the disc is experiencing the environmental vibration, which should cause the disc surface moving up and down. Still further, a need exists for an optical interferometer that permits adjustment of the separation distance between the two beams as they strike the target surface.